On such vehicles, each track is guided firstly over ground wheels, secondly over a drive member referred to as a sprocket wheel which is generally situated at the rear end of the vehicle and above the ground wheels, and thirdly over a return pulley which is generally situated at the front end of the vehicle and at the same height as the sprocket wheel.
The ground wheels support the chassis of the vehicle via a suspension system whose up-and-down motion causes the height above ground of the sprocket wheel and of the return pulley to vary. Such variations in height give rise to variations in track tension, thereby giving rise to slack in certain portions of the tracks. The braking forces and the acceleration forces that are applied to the tracks via the sprocket wheels also give rise to slack in certain portions of the tracks.
Naturally, a certain amount of pre-stress or initial tension may be given to the tracks in an attempt to prevent,. at least in part, slack from occurring in certain portions of the tracks. For that purpose, the return pulley may be associated with a hydraulic actuator for displacing the pulley, the actuator being fed from a source of liquid under pressure. The tension of the track that runs over the return pulley is adjusted by adjusting the pressure in the actuator.
Unfortunately, high initial track tension gives rise to greater track fatigue and to greater track wear when the vehicle is in motion, thereby putting a limit on the maximum value that can be given to the initial tension. Furthermore, when forces that are much greater than the initial tension are transmitted to the tracks via the drive system of the vehicle, such forces pass through the taut lengths of the tracks, thereby compressing the suspension of the vehicle and giving rise to slack in the opposite lengths of the tracks.
The following two drawbacks result from slack occurring in certain portions of the tracks when the vehicle is in motion:
the track might ride over the drive sprocket wheel during braking, the slack occurring in the rear length of the track then tending to ride up over the teeth of the sprocket wheel, thereby causing a shoe of the track to skip a sprocket, resulting in very jerky transmission; and
the track might come off on soft and uneven terrain, where earth might exert forces laterally on the slack lengths of the tracks, which forces, together with the absence of tension, might cause the tracks to become disengaged from their side guides, thereby causing them to come off.
To mitigate those drawbacks, proposals have already been made to associate the return pulley with the first ground wheel (or the wheel that is closest to the pulley) via a mechanical or hydromechanical coupling system which is designed to take up the slack in the track by moving the return pulley forwards when the front wheel rises relative to the chassis of the vehicle, so as to keep the track under tension. Such a system offers the advantage of being passive, and of not consuming any energy, but it only solves part of the problem, by dealing with slack in the front length of the track only. Furthermore, that system can result in dynamic forces that prevent the suspension of the vehicle from operating. To keep the track under tension, the return pulley must be moved forwards very quickly when the front ground wheel rises suddenly (when the vehicle runs over an obstacle), and this means that the return pulley and the front ground wheel must be dynamically united, and that extremely large forces may be caused in the coupling system between the return pulley and the front wheel.
Proposals have also been made to servo-control the position of the return pulley to the state of the suspension of the vehicle, by means of a hydraulic actuator for displacing the return pulley, and of a servo-control circuit comprising a computer and geometrical-deformation sensors mounted on the suspension, thereby constituting an "active" system. However, since the hydraulic actuator must be dimensioned so as to withstand the maximum braking force that can be applied to the track, the power consumed by such an active system is very high, e.g. about several hundred kilowatts per track.